Co-pending application Ser. No. 892,271 entitled "POROUS HIGHLY EXPANDED FLUOROPOLYMERS AND PROCESS THEREFOR", incorporated herein by reference, discloses the use of elastomers which strengthen expanded poly(tetrafluoroethylene) fibrils by forming a continuous matrix interpenetrating the microstructure of the fibrils. In so doing, it renders the poly(tetrafluoroethylene) structure porous but yet durable with excellent pliability for use as a vascular graft. More importantly, however, addition of an elastomer to the poly(tetrafluoroethylene) allows an implant or preferably, a vascular graft/made from the material to be biologically compatible with surrounding tissue.
This invention relates to a multi-layered polytetrafluoroethylene/elastomer composite structure that can be formed into an implant where there is an improvement in the luminal hydrophobicity, suturability, compliance, strength and elasticity due to the novel arrangement of respective layers of poly(tetrafluoroethylene), polytetrafluoroethylene/elastomer and elastomer. This invention relates to a composite structures, more specifically composite medical devices for in vivo implantation, such as heart valve leaflets, sutures, vascular access devices or any related products, but more particularly relates to vascular grafts.
Conventional vascular grafts manufactured from porous poly(tetrafluoroethylene) have limitations in their strength and compliance. The porous grafts do not hold or resist dilation unless reinforced in some manner. For example, vascular grafts have been reinforced by incorporating in the tube wall external ribs, see U.S. Pat. No. 4,550,447, issued to Seiler, Jr. et al on Nov. 5, 1985. Other methods of reinforcing vascular grafts include wrapping the vascular graft with a reinforcing film for support. This reinforcement slows down the tissue ingrowth preventing rapid healing. This is because of the relatively low radial tensile strength of poly(tetrafluoroethylene). In addition, the grafts are stiff and non-compliant to the natural artery.
Laminated vascular grafts have also been proposed with the laminated materials bonded in a manner to place porous, compacted poly(tetrafluoroethylene) in a position to be in contact with the blood surrounded by a layer of a suitable biocompatible material so that the implant allegedly may be accepted by the surrounding tissue. U.S. Pat. No. 4,576,608 describes a vascular graft having two layers, an inner layer comprising a blend of poly(tetrafluoroethylene) fibers and resin having a specific porosity wherein the outer layer comprises a fused blend of poly(tetrafluoroethylene) fibers and carbon fibers or silicone rubber. Other suitable biocompatible materials used in the lamination may be Teflon FEP, manufactured by DuPont Company or other biocompatible fabrics such as polyamide, polyaramid, polyimide or polyester fabric.
In U.S. Pat. No. 4,286,341, issued to Greer et al on Sept. 1, 1981, a vascular graft is disclosed which is formed from a substantially non-thrombogenic hydrogel, with the luminal surface having relatively small pores or microvoids suitable for tissue ingrowth, while the outer surface has a heterogenous microstructure including relatively large pores or macrovoids especially suitable for cellular ingrowth.
U.S. Pat. No. 4,321,711 discloses a vascular prosthesis comprising porous tubing of poly(tetrafluoroethylene) containing an anti-coagulant substance and bonded to its outside surface, a porous elastomer coating containing a substance which counteracts the anti-coagulant. Typically, the anti-coagulant substance is heparin. Any heparin antagonist such as protamine may be used in the elastomer coating to counteract the heparin. The elastomer is typically fluorine rubber, silicone rubber, etc. While the implants taught in the above discussed references may be porous and flexible, they do not provide the strength, elasticity or biological compatibility of the natural artery.
Other types of reinforced vascular grafts are taught in U.S. Pat. Nos. 4,193,138, issued to Okita on March 18, 1980., and 4,229,838, issued to Mano on Oct. 28, 1980. The vascular graft taught in Okita is a composite structure of a porous polytetrafluoroethylene tube in which the pores have been filled in with at least one water-insolubilized water-soluble polymer. Mano also teaches a composite vascular graft structure including a porous polytetrafluoroethylene tubing with a polyethyleneimine in the pores.
In U.S. Patent No. 4,718,907, issued to Karwoski et al on Jan. 12, 1988 a process is disclosed for depositing a fluorine-containing coating is disclosed. The process obtains the fluorine-containing coating by passing a polymerizable fluorine-containing gas through a tubular substrate, while a radio frequency field is being applied to cause the deposition and polymerization of the fluorine-containing gas on to the exposed surfaces of the substrate. The resulting product may be useful for preparing a vascular graft.
A process for preparing high strength porous polytetrafluoroethylene products is disclosed in U.S. Pat. No. 4,482,516, issued to Bowman et al on Nov. 13, 1984. The disclosed process involves densifying a polytetrafluoroethylene material to achieve the higher strength.
While the above described vascular grafts, or processes of preparing the same, provide for a stronger graft, such grafts do not possess the desired porosity necessary to achieve the requisite tissue ingrowth.
Another suggested approach for reinforcing vascular grafts is disclosed in U.S. Pat. No. 4,306,318, issued to Mano et al on Dec. 22, 1981. The disclosed vascular graft is prepared by helically wrapping an elastic fiber about the exterior surface of a porous tubing of polytetrafluoroethylene. The elastic fibers are wrapped about the exterior of the tubing and the adhered thereto. The strength imparted to the resulting vascular graft is dependent upon the tensile strength of the elastic fibers being used. This reinforcement is constant across the vascular graft.
There is a need for an in vivo implantable medical device, and in particular vascular grafts which are formed as a composite structure that mimics the natural artery composition of collagen and elastin and is acceptable to the surrounding tissue.